1. Field of the Invention
The present invention relates to a pickup device for writing or reading information on or from an optical disk.
2. Description of the Related Background Art
When a recording layer surface of an optical disk is directly exposed, the recording layer surface is flawed or is damaged due to collision with a pickup head at the time that information is written on or read from the optical disk. Since recording or playing process become disabled in the case, the optical disk has a transmission substrate of a predetermined thickness to write or read via the transmission substrate. Although the substrate for the purpose is formed, for example, by injection molding, it is difficult to make the substrate thickness equal to a determined value across the entire surface and normally, a thickness error of a few dozen xcexcm occurs. In a conventional apparatus for reading or writing, the numerical aperture of an objective lens is determined not to occur any problem even when such a thickness error is included. However, due to demands for greater information amounts and higher recording densities, an increase in the numerical aperture of the objective lens is considered so as to enable recording with a reduced spot diameter on the disk. In such cases, arrangements are made to correct spherical aberration that is generated by the thickness error of the transmission substrate. For example, as disclosed in Japanese Laid-open Patent Publication No. 10(1998)-106012, the modulation level, jitter, and error rate of a signal reproduced from the disk are monitored, and a spherical aberration correction means (lens) is driven in response to the monitored results.
However, the spherical aberration cannot be corrected by the above method in the case of recording onto an unrecorded disk, since a reproduced signal cannot be obtained. Furthermore, the jitter, error rate, modulation level, etc., are also affected, for example, by a tilt, birefringence, etc., of the substrate, and when such factors exist, the jitter, error rate, modulation level, etc., increase, causing a servo system to operate erroneously.
It is, therefore, an object of the present invention to provide a pickup device which can correct spherical aberration issued by a thickness error of a transmission substrate regardless of whether a disk is recorded or unrecorded even if an optical system includes an objective lens having high numerical aperture.
According to the present invention, there is a pickup device for writing or reading information onto or from an optical disk having a recording surface that is covered with a transmission substrate. The pickup device comprises: light extraction means for illuminating a light beam onto the recording surface via an objective lens with a first predetermined numerical aperture and for obtaining light reflected from the recording surface via the objective lens; first focus error detection means for detecting first reflected light corresponding to first illuminated light which is illuminated via a portion with a numerical aperture that is less than or equal to a second predetermined numerical aperture which is smaller than the first predetermined numerical aperture, of the reflected light obtained from the light extraction means, and for generating a first error signal which indicates a focus error of the first illuminated light on the recording surface; second focus error detection means for detecting second reflected light corresponding to second illuminated light which is illuminated via a portion with a numerical aperture that is less than or equal to a predetermined numerical aperture which is greater than the second predetermined numerical aperture, of the reflected light obtained from the light extraction means, and for generating a second error signal which indicates a focus error of the second illuminated light on the recording surface; and means for obtaining a signal corresponding to spherical aberration by using at least one of the first or second error signal.
In the pickup device according to the invention, since a first reflected light, of the reflected light from the recording surface of the optical disk, which has been transmitted through a portion of the objective lens that is of no more than a second predetermined numerical aperture which is smaller than a first predetermined numerical aperture, is detected to generate a first error signal and a second reflected light, of the reflected light, which has been transmitted through a portion of the objective lens that is of no more than a predetermined numerical aperture, which is greater than the second predetermined numerical aperture, is detected to generate a second error signal, at least one of the first and second error signal can be used to obtain a signal that indicates the spherical aberration caused by the thickness error of the transmission substrate.